Resonant inverter circuit

ABSTRACT

A resonant inverter circuit is provided which can be made lighter in weight and smaller in capacity. With a main circuit in steady mode, IGBT of an auxiliary circuit are controlled and the energy of an electric current is stored in a resonant inductance. Next, snubber capacitors are charged and discharged by means of an electric current. At this time, the voltages across both terminals of IGBT corresponding with the snubber capacitors being charged becomes zero, and ZVS is achieved. Furthermore, when the snubber capacitors being discharged have been completely discharged, and free wheeling diodes and IGBT corresponding with these snubber capacitors conduct, both the voltages across both terminals of the IGBT and the electric current become zero and ZVS and ZCS are achieved.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an inverter circuit for drivinga load such as a motor, and relates particularly to a resonant invertercircuit comprising a snubber capacitor for performing soft switching.

[0003] 2. Description of the Related Art

[0004] Examples of conventional inverter circuits for driving a loadsuch as a motor include the technology disclosed in U.S. Pat. No.5,710,698, U.S. Pat. No. 5,642,273 and U.S. Pat. No. 5,047,913. Forexample, as shown in FIG. 8, in a soft switching inverter according toconventional technology, a motor 1 comprising a three phase inductionmotor or a DC brushless motor or the like is connected to the softswitching inverter as a load, and comprises, for example, an inverterusing IGBT (Insulated Gate Bipolar Transistor) Q1 to Q6 as switchingelements.

[0005] In the inverter, the IGBT Q1 to Q6 are connected to both sides ofa DC power source 3 in a three phase bridge structure comprising a Uphase, a V phase and a W phase. Free wheeling diodes (FWD) D1 to D6 areconnected between a collector terminal and an emitter terminal of eachIGBT for the purpose of circulating the regenerative energy produced bythe inductive load of the motor 1 and the electric current energy storedby the inductive load. Furthermore, snubber capacitors C1 to C6 forabsorbing the surge voltage applied between the collector terminal andthe emitter terminal of the IGBT during turn-on or turn-off areconnected between the collector terminal and the emitter terminal ofeach IGBT.

[0006] In addition, the DC power source 3 and a smoothing capacitor C9are connected to the inverter. Mid-point voltage storage capacitors C7and C8 for storing a mid-point voltage are connected in series to bothsides of the smoothing capacitor C9. An inductance L1 which resonateswith the snubber capacitors C1 and C2, and a bi-directional switchingunit SU1 for channeling the resonant current via the inductance L1 areconnected between the connection point of the mid-point voltage storagecapacitors C7 and C8 and the connection point of the snubber capacitorsC1 and C2 of the U phase. In a similar manner, an inductance L2 whichresonates with the snubber capacitors C3 and C4, and a bi-directionalswitching unit SU2 for channeling the resonant current via theinductance L2 are connected between the connection point of themid-point voltage storage capacitors C7 and C8 and the connection pointof the snubber capacitors C3 and C4 of the V phase. In addition, aninductance L3 which resonates with the snubber capacitors C5 and C6, anda bi-directional switching unit SU3 for channeling the resonant currentvia the inductance L3 are connected between the connection point of themid-point voltage storage capacitors C7 and C8 and the connection pointof the snubber capacitors C5 and C6 of the W phase.

[0007] A configuration as shown above may also be called an auxiliaryresonant commutated arm link type snubber inverter. In such a softswitching inverter, if for example the IGBT Q1 is turned off, and theIGBT Q2 is then turned on after a short delay, the charging current ofthe snubber capacitor C1 and the discharging current of the snubbercapacitor C2 flow through the mid-point voltage storage capacitors C7and C8 via the inductance L1. At the same time, if the IGBT Q4 and Q6are turned off, and the IGBT Q3 and Q5 are then turned on after a shortdelay, the charging current of the snubber capacitors C4 and C6 and thedischarging current of the snubber capacitors C3 and C5 are suppliedfrom the mid-point voltage storage capacitors C7 and C8 via theinductance L2 and L3.

[0008] By charging and discharging the snubber capacitor according tothe resonant current of the snubber capacitor and the inductance in thismanner, when the IGBT is turned off and the snubber capacitor ischarged, because the rise in the voltage applied to the IGBT is delayedaccording to the time constant applied by the snubber capacitor, ZVS(Zero Voltage Switching) of the IGBT can be realized. Conversely, if thesnubber capacitor is discharged before the IGBT is turned on, a freewheeling diode conducts and the voltage and current applied to the IGBTbecomes zero, thereby realizing ZVS (Zero Voltage Switching) and ZCS(Zero Current Switching) of the IGBT. Consequently, the loss whichoccurs during turn-on and turn-off of the switching elements such as theIGBTs, can be reduced.

[0009] Furthermore, FIG. 9 also shows a soft switching inverteraccording to conventional technology, which may also be called anauxiliary resonant AC link snubber inverter. In a similar manner as inthe auxiliary resonant commutated arm link type snubber inverter shownin FIG. 8, a smoothing capacitor C9 and the inverter are connected toboth sides of a DC power source 3. In the inverter, the IGBTs Q1 to Q6,to which are connected free wheeling diodes D1 to D6 and snubbercapacitors C1 to C6 respectively, are connected in a three phase bridgestructure comprising a U phase, a V phase and a W phase. An inductanceL4 which resonates with the snubber capacitors C1 and C2, and abi-directional switching unit SU4 for channeling the resonant currentvia the inductance L4 are connected between the connection point of thesnubber capacitors C1 and C2 of the U phase of the inverter and theconnection point of the snubber capacitors C3 and C4 of the V phase ofthe inverter. Furthermore, an inductance L5 which resonates with thesnubber capacitors C3 and C4, and a bi-directional switching unit SU5for channeling the resonant current via the inductance L5 are connectedbetween the connection point of the snubber capacitors C3 and C4 of theV phase of the inverter and the connection point of the snubbercapacitors C5 and C6 of the W phase of the inverter. In addition, aninductance L6 which resonates with the snubber capacitors C5 and C6, anda bi-directional switching unit SU6 for channeling the resonant currentvia the inductance L6 are connected between the connection point of thesnubber capacitors C1 and C2 of the U phase of the inverter and theconnection point of the snubber capacitors C5 and C6 of the W phase ofthe inverter.

[0010] The only difference between the auxiliary resonant AC linksnubber inverter shown in FIG. 9 and the auxiliary resonant commutatedarm link type snubber inverter shown in FIG. 8 is the path of theelectric current for charging and discharging the snubber capacitors,and the principles involved in achieving ZVS and ZCS at each of the IGBTswitching elements are the same.

[0011] In a soft switching inverter according to the above conventionaltechnology, the electric current which flows through the IGBT (theswitching elements) and the voltage applied to the IGBT can becontrolled by forming a resonant circuit comprising the snubbercapacitor and each inductance. Consequently, this is effective inreducing the loss which occurs in the switching elements during turn-onor turn-off.

[0012] However, because the core capacity required for the inductance isdetermined by the peak conducted current, as the controlled load currentincreases, the weight and capacity of the inductance also increases.Consequently, a problem arises in that a soft switching inverteraccording to conventional technology, which requires three inductanceswith an electric current which is at least as large as the load current,cannot be made smaller or lighter due to the increase in weight andcapacity required for the inductances.

SUMMARY OF THE INVENTION

[0013] In consideration of the above circumstances, an object of thepresent invention is to provide a resonant inverter circuit that can bemade lighter in weight and smaller in capacity.

[0014] In order to resolve the above problems, a resonant snubberinverter circuit according to the present invention comprises: six mainswitching elements (such as IGBT Q1 to Q6 of the embodiment) whicheither conduct or are cutoff by means of switching control, whereinthree sets of two main switching elements which comprise each phase of athree phase bridge are connected in the three phase bridge, and each setof the main switching elements is connected in series to both terminalsof a power source (such as the DC power source 3 of the embodiment); sixfree wheeling diodes (such as the free wheeling diodes D1 to D6 of theembodiment) connected in parallel between two terminals of each of themain switching elements; six snubber capacitors (such as the snubbercapacitors C1 to C6 of the embodiment) connected in parallel between twoterminals of each of the main switching elements; a three phase outputterminal for connecting a load (such as the motor 1 of the embodiment),connected respectively to a connection point of the two main switchingelements comprising each of the sets; a bridge circuit having sixauxiliary switching elements (such as IGBT Q7 to Q12, and the protectiondiodes D7 to D12 of the embodiment) for causing an electric current toflow in a single direction, wherein three sets of two auxiliaryswitching elements are connected in a three phase bridge, and connectionpoints common to the two auxiliary switching elements which compriseeach set of the auxiliary switching elements are connected respectivelyto the three phase output terminal; and a resonant inductance (such asthe resonant inductance Lr of the embodiment) forming a resonant circuitwith the snubber capacitors, connected to an opposite terminal to aterminal connected to the connection point of the auxiliary switchingelements.

[0015] In a soft switching inverter of the above structure, the chargingand discharging of the six snubber capacitors is controlled by theresonant electric current flowing to the single inductance, which formsa resonant circuit with the six snubber capacitors which are connectedin parallel to the six main switching elements, and the bridge circuitcomprising the six auxiliary switching elements which are connected tothe inductance. Consequently, whereas in the conventional technology oneinductance is required for each phase producing a total of threeinductances, in the present invention this number is reduced to oneinductance across the entire circuit, making it possible to perform softswitching with less switching loss in the inverter circuit, and operatethe resonant inverter circuit more efficiently. Consequently, theinverter can be made lighter in weight and smaller in capacity.

[0016] In a resonant snubber inverter circuit of the present invention,it is preferable that one of the auxiliary switching elements of eachset of the auxiliary switching elements includes a unidirectionalswitching element (such as IGBT Q7, Q9, Q11 of the embodiment) whichonly conducts electric current in a direction flowing into theconnection point of the auxiliary switching elements, and anotherauxiliary switching element of each set of the auxiliary switchingelements includes a unidirectional switching element (such as IGBT Q8,Q10, Q12 of the embodiment) which only conducts electric current in adirection flowing out from the connection point of the auxiliaryswitching elements.

[0017] According to the above construction, using an inductance electriccurrent which flows in one direction, those two snubber capacitors amongthe total of 6 snubber capacitors which are connected in series to theboth sides of the power source are deemed to be one set of snubbercapacitors comprising one phase of the three phase inverter, and thecharging and discharging current of the snubber capacitors of a phasewhich flows in the opposite direction is deemed to be the charging anddischarging current of snubber capacitors of another phase, and so forall the phase combinations in a circuit formed by a three phase bridgeconnection, it becomes possible to control the direction of electriccurrent flow through each phase.

[0018] In a resonant snubber inverter circuit of the present invention,it is preferable that the unidirectional switching elements be elementshaving a withstand voltage greater than a power source voltage (such asthe power source voltage VB of the embodiment) of the power source, inboth forward and backward directions.

[0019] According to the above construction, it becomes possible forswitching control to be performed on the auxiliary switching elements sothat all of the charging and discharging electric current patterns ofthe snubber capacitors can be produced.

[0020] In a resonant snubber inverter circuit of the present invention,it is preferable that the unidirectional switching elements be insulatedgate bipolar transistors (such as the IGBT Q7 to Q12 of the embodiment),each auxiliary switching element comprises the insulated gate bipolartransistor and a diode (such as the protection diodes D7 to D12 of theembodiment), and the diode is either one of a diode in which an anodeterminal is connected to a collector terminal of the insulated gatebipolar transistor, and a diode in which a cathode terminal is connectedto an emitter terminal of the insulated gate bipolar transistor.

[0021] According to the above construction, high speed voltage-drivenswitching becomes possible using the control voltage applied to thecontrol terminal of the IGBT. In addition, the characteristics of theIGBT allow switching in which the saturation voltage between theterminals during conduction is low.

[0022] In a resonant snubber inverter circuit of the present invention,it is preferable that the unidirectional switching elements are metaloxide semiconductor field effects transistors, each auxiliary switchingelement comprises the metal oxide field effects transistor and a diode,and the diode is either one of a diode in which an anode terminal isconnected to a drain terminal of the metal oxide field effectstransistor, and a diode in which a cathode terminal is connected to asource terminal of the metal oxide field effects transistor.

[0023] According to the above construction, the characteristics of theMOSFET enable high speed voltage-driven switching by using the controlvoltage applied to the control terminal of the MOSFET.

[0024] In a resonant snubber inverter circuit of the present invention,it is preferable that the unidirectional switching elements are reverseblocking thyristors (such as the reverse blocking thyristors T1 to T6 ofthe embodiment).

[0025] According to the above construction, the characteristics of thethyristors enable current-driven switching of large currents by usingthe control current applied to the control terminal of the reverseblocking thyristors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 is a circuit diagram showing the construction of aninverter circuit according to an embodiment of the present invention.

[0027]FIG. 2A shows the operation of a mode 1 of the inverter circuitaccording to the same embodiment.

[0028]FIG. 2B shows the operation of a mode 2 of the inverter circuitaccording to the same embodiment.

[0029]FIG. 2C shows the operation of a mode 3 of the inverter circuitaccording to the same embodiment.

[0030]FIG. 3A shows the operation of a mode 4 of the inverter circuitaccording to the same embodiment.

[0031]FIG. 3B shows the operation of a mode 5 of the inverter circuitaccording to the same embodiment.

[0032]FIG. 3C shows the operation of a mode 6 of the inverter circuitaccording to the same embodiment.

[0033]FIG. 4A shows the operation of a mode 7 of the inverter circuitaccording to the same embodiment.

[0034]FIG. 4B shows the operation of a mode 8 of the inverter circuitaccording to the same embodiment.

[0035]FIG. 4C shows the operation of a mode 9 of the inverter circuitaccording to the same embodiment.

[0036]FIG. 5A shows the operation of a mode 10 of the inverter circuitaccording to the same embodiment.

[0037]FIG. 5B shows the operation of a mode 11 of the inverter circuitaccording to the same embodiment.

[0038]FIG. 6 is a waveform diagram showing the changing waveforms ofeach portion for each mode of the inverter circuit according to the sameembodiment.

[0039]FIG. 7 is a circuit diagram showing the inverter circuit accordingto another embodiment, wherein reverse blocking thyristors are used asthe auxiliary switching elements.

[0040]FIG. 8 is a circuit diagram showing the construction of aninverter circuit according to conventional technology.

[0041]FIG. 9 is a circuit diagram showing another construction of aninverter circuit according to conventional technology.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] As follows is a description of preferred embodiments of thepresent invention, with reference to the drawings.

[0043]FIG. 1 is a circuit diagram of an inverter circuit according to anembodiment of the present invention. In FIG. 1, the inverter circuitaccording to the present embodiment comprises a main circuit 2A to whicha motor 1 comprising a three phase induction motor or a DC brushlessmotor or the like is connected as a load, and an auxiliary circuit 2B.The main circuit 2A comprises an inverter using, for example, IGBT Q1 toQ6 as the main switching elements. The auxiliary circuit 2B comprisesauxiliary switching elements using, for example, IGBT Q7 to Q12 asunidirectional switching elements, and a resonator comprising a resonantinductance Lr. Moreover, IGBT Q7 to Q12 have a withstand voltage equalto or greater than the power source voltage VB of the DC power source 3in both the forward and backward directions. Other elements such asreverse blocking thyristors, GTO (Gate Turn Off thyristors), bipolartransistors or MOSFET (Metal Oxide Semiconductor Field EffectTransistor) may also be used as the switching elements instead of theIGBTs used in Q1 to Q12.

[0044] Furthermore, the main circuit 2A is a circuit wherein IGBT Q1 toQ6 are connected in a three phase bridge structure, comprising a Uphase, a V phase and a W phase, to both ends of a smoothing capacitorC9, which is connected in parallel to the DC power source 3. Freewheeling diodes D1 to D6 are connected between the collector terminaland the emitter terminal of each IGBT in order to circulate theregenerative energy produced by the inductive load of the motor 1, andthe current energy stored in the inductive load. Specifically, thecollector terminal of each IGBT is connected to the anode terminal of afree wheeling diode, and the emitter terminal of the IGBT is connectedto the cathode terminal of the free wheeling diode, respectively.Furthermore, snubber capacitors C1 to C6 for absorbing the surge voltageapplied between the collector terminal and the emitter terminal of anIGBT during turn-on and turn-off of the IGBT are also connected betweenthe collector terminal and the emitter terminal of each IGBT Q1 to Q6,respectively.

[0045] Furthermore within the main circuit 2A, three phase outputterminals for the U phase, the V phase and the W phase of the invertercircuit of the present embodiment extend from the connection points ofthe emitter terminal of the IGBT Q1 and the collector terminal of theIGBT Q2, the emitter terminal of the IGBT Q3 and the collector terminalof the IGBT Q4, and the emitter terminal of the IGBT Q5 and thecollector terminal of the IGBT Q6. The terminals of the U phase, the Vphase and the W phase of the motor 1 are connected respectively to thesethree phase output terminal. In addition, the auxiliary circuit 2B isalso connected to the three phase output terminals of the main circuit2A.

[0046] In the auxiliary circuit 2B, the IGBT Q7 to Q12 are connected ina three phase bridge structure comprising a U′ phase, a V′ phase and aW′ phase to both ends of the resonant inductance Lr, which forms aresonant circuit with the snubber capacitors C1 to C6 used in the maincircuit 2A. Protection diodes D7, D9 and D11 are connected to thecollector terminals of the IGBT Q7, Q9 and Q11 respectively. Morespecifically, the collector terminals of the IGBT Q7, Q9 and Q11 areconnected in series to the anode terminals of the protection diodes D7,D9 and D11 respectively. In a similar manner, protection diodes D8, D10and D12 are connected to the emitter terminals of the IGBT Q8, Q10 andQ12 respectively. More specifically, the emitter terminals of the IGBTQ8, Q10 and Q12 are connected in series to the cathode terminals of theprotection diodes D8, D10 and D12, respectively.

[0047] Moreover, here the series circuits comprising the unidirectionalswitching elements and the protection diodes are defined as auxiliaryswitching elements.

[0048] Furthermore as described above, the protection diodes D7, D9 andD11 are connected to the collector terminals of the IGBT Q7, Q9 and Q11,and the protection diodes D8, D10 and D12 are connected to the emitterterminals of the IGBT Q8, Q10 and Q12. However, the protection diodesD7, D9 and D11 may also be connected to the emitter terminals of theIGBT Q7, Q9 and Q11 and the protection diodes D8, D10 and D12 may alsobe connected to the collector terminals of the IGBT Q8, Q10 and Q12.Furthermore, the protection diodes could also all be connected to thecollector terminals of the IGBT Q7 to Q12, or conversely, the protectiondiodes could be all connected to the emitter terminals of the IGBT Q7 toQ12. In brief, any configuration is suitable, provided the IGBTs areprotected by the protection diodes from the voltage applied to theauxiliary switching elements incorporating the IGBTs.

[0049] In addition, the same applies in those cases where MOSFET areused instead of IGBT. In other words, the MOSFET should be protectedfrom the voltage applied to the auxiliary switching elementsincorporating the MOSFETs, by either connecting the anode terminal ofthe protection diode to the drain terminal of the MOSFET in series, orconnecting the cathode terminal of the protection diode to the sourceterminal of the MOSFET in series.

[0050] Furthermore, when reverse blocking thyristors are used in theunidirectional switching elements, protection diodes are not needed inthe auxiliary switching elements. Moreover, the construction employedwhen reverse blocking thyristors are used in the unidirectionalswitching elements is described below in detail.

[0051] Furthermore, the three phase output terminals of the main circuit2A, and the auxiliary circuit 2B are connected in the following manner.Namely, the U phase terminal of the three phase output terminal isconnected to the connection point in the U′ phase of the auxiliarycircuit 2B (in other words, the connection point between the auxiliaryswitching element incorporating the IGBT Q7 and the auxiliary switchingelement incorporating the IGBT Q8). In a similar manner, the V phaseterminal of the three phase output terminal is connected to theconnection point in the V′ phase of the auxiliary circuit 2B (in otherwords, the connection point between the auxiliary switching elementincorporating the IGBT Q9 and the auxiliary switching elementincorporating the IGBT Q10). In addition, the W phase terminal of thethree phase output terminal is connected to the connection point in theW′ phase of the auxiliary circuit 2B (in other words, the connectionpoint between the auxiliary switching element incorporating the IGBT Q11and the auxiliary switching element incorporating the IGBT Q12).

[0052] More specifically, in the circuit structure shown in FIG. 1, theU phase terminal of the three phase output terminal is connected to theconnection point between the emitter terminal of the IGBT Q7 and thecollector terminal of the IGBT Q8. In a similar manner, the V phaseterminal of the three phase output terminal is connected to theconnection point between the emitter terminal of the IGBT Q9 and thecollector terminal of the IGBT Q10. In addition, the W phase terminal ofthe three phase output terminal is connected to the connection pointbetween the emitter terminal of the IGBT Q11 and the collector terminalof the IGBT Q12.

[0053] Moreover, when the connections of the IGBT and the protectiondiodes, which comprise the switching elements, are reversed, then thenames of the appropriate terminals for connection are also substituted.

[0054] Next, the operation of the inverter circuit according to thepresent embodiment will be described using the drawings. In describingthe operation of the circuit, first the symbols for defining the voltageand the electric current of each portion of the circuit diagram of FIG.1 and the on/off switching of each switching element will be described.

[0055] Firstly, the voltage and electric current of each portion isdefined in the manner described below.

[0056] (1) With the collector side of the IGBT Q1 defined as the forwarddirection, the voltage applied to both ends of the parallel circuitcomprising the IGBT Q1, the free wheeling diode D1 and the snubbercapacitor C1 is defined as V1. Furthermore, the electric current flowingfrom this parallel circuit towards the load (the motor 1) is defined asIs1, with this flow direction defined as the forward direction.

[0057] (2) In a similar manner, with the collector side of the IGBT Q3defined as the forward direction, the voltage applied to both ends ofthe parallel circuit comprising the IGBT Q3, the free wheeling diode D3and the snubber capacitor C3 is defined as V3. Furthermore, the electriccurrent flowing from this parallel circuit towards the load is definedas Is3, with this flow direction defined as the forward direction.

[0058] (3) With the collector side of the IGBT Q5 defined as the forwarddirection, the voltage applied to both ends of the parallel circuitcomprising the IGBT Q5, the free wheeling diode D5 and the snubbercapacitor C5 is defined as V5. Furthermore, the electric current flowingfrom this parallel circuit towards the load is defined as Is5, with thisflow direction defined as the forward direction.

[0059] Furthermore, electric currents Is2, Is4 and Is6, for which thedefinitions for forward and reverse current directions are opposite tothose of the electric currents Is1, Is3 and Is5 described in (1) to (3)above, and voltages V2, V4 and V6 are defined as follows.

[0060] (4) With the collector side of the IGBT Q2 defined as the forwarddirection, the voltage applied to both ends of the parallel circuitcomprising the IGBT Q2, the free wheeling diode D2 and the snubbercapacitor C2 is defined as V2. Furthermore, the electric current flowingfrom the load towards this parallel circuit is defined as Is2, with thisflow direction defined as the forward direction.

[0061] (5) In a similar manner, with the collector side of the IGBT Q4defined as the forward direction, the voltage applied to both ends ofthe parallel circuit comprising the IGBT Q4, the free wheeling diode D4and the snubber capacitor C4 is defined as V4. Furthermore, the electriccurrent flowing from the load towards this parallel circuit is definedas Is4, with this flow direction defined as the forward direction.

[0062] (6) With the collector side of the IGBT Q6 defined as the forwarddirection, the voltage applied to both ends of the parallel circuitcomprising the IGBT Q6, the free wheeling diode D6 and the snubbercapacitor C6 is defined as V6. Furthermore, the electric current flowingfrom the load towards this parallel circuit is defined as Is6, with thisflow direction defined as the forward direction.

[0063] In addition, when current flow into the load is defined as theforward direction, the three phase electric currents which flow only tothe load are defined as Iu, Iv and Iw respectively.

[0064] Furthermore, the on/off state of the IGBT Q1 to Q12 is defined inthe following manner, using a logical value “1”/“0”.

[0065] Firstly, a state where the upper IGBT Q1 of the U phase of themain circuit 2A is on and the lower IGBT Q2 is off, is represented as“1”, and a state where the upper IGBT Q1 of the U phase is off and thelower IGBT Q2 is on is represented as “0”. In a similar manner, a statewhere the upper IGBT Q3 of the V phase is on and the lower IGBT Q4 isoff is represented as “1”, and a state where the upper IGBT Q3 of the Vphase is off and the lower IGBT Q4 is on is represented as “0”.Likewise, in the W phase, a state where the upper IGBT Q5 of the W phaseis on and the lower IGBT Q6 is off is represented as “1”, and a statewhere the upper IGBT Q5 of the W phase is off and the lower IGBT Q6 ison is represented as “0”.

[0066] Furthermore, a state where the upper IGBT Q7 of the U′ phase ofthe auxiliary circuit 2B is on and the lower IGBT Q8 is off isrepresented as “1”, and a state where the lower IGBT Q8 of the U′ phaseis on and the upper IGBT Q7 is off is represented as “0”. In a similarmanner, a state where the upper IGBT Q9 of the V′ phase is on and thelower IGBT Q10 is off is represented as “1”, and a state where the lowerIGBT Q10 of the V′ phase is on and the upper IGBT Q9 is off isrepresented as “0”. Likewise, in the W′ phase, a state where the upperIGBT Q11 of the W′ phase is on and the lower IGBT Q12 is off isrepresented as “1”, and a state where the lower IGBT Q12 of the W′ phaseis on and the upper IGBT Q11 is off is represented as “0”.

[0067] Accordingly, for example, (U, V, W)=(1, 0, 0) indicates a statewhere the IGBT Q1 is on, the IGBT Q2 is off, the IGBT Q3 is off, theIGBT Q4 is on, the IGBT Q5 is off, and the IGBT Q6 is on.

[0068] Moreover, in the layout of components shown in FIG. 1, the upperIGBT Q1, Q3, Q5, Q7, Q9 and Q11 are defined as “H” side switchingelements, and the lower IGBT Q2, Q4, Q6, Q8, Q10 and Q12 are defined as“L” side switching elements.

[0069] Furthermore, a state where both the H and L side IGBT are offcannot be shown by “0” or “1”. An annotation for this state “H, Lsimultaneously off” is added to the waveform diagram of FIG. 6 as anintermediate level between “1” and “0”.

[0070] In addition, the operation of each mode from mode 1 to mode 11 asshown in FIG. 2A to FIG. 5B is defined below. Here, the case where (U,V, W) are controlled so that (1, 0, 0)−>(0, 0, 1)−>(1, 1, 0) is taken asan example for describing the control mode of the inverter circuitaccording to the present embodiment. A summary of the operation (state)of the modes 1 to 11 for this particular case is shown below.

[0071] (a) Mode 1: Steady mode where (U, V, W)=(1, 0, 0).

[0072] (b) Mode 2: Initial electric current storage mode in a transitionstate from (1, 0, 0) to (0, 0, 1).

[0073] (c) Mode 3: Resonant mode in the transition state from (1, 0, 0)to (0, 0, 1).

[0074] (d) Mode 4: Regenerative mode in the transition state from (1, 0,0) to (0, 0, 1).

[0075] (e) Mode 5: Steady mode where (U, V, W)=(0, 0, 1)

[0076] (f) Mode 6: Initial electric current storage mode in a transitionstate from (0, 0, 1) to (1, 1, 0) (step 1).

[0077] (g) Mode 7: Initial electric current storage mode in thetransition state from (0, 0, 1) to (1, 1, 0) (step 2).

[0078] (h) Mode 8: Resonant mode in the transition state from (0, 0, 1)to (1, 1, 0).

[0079] (i) Mode 9: Regenerative mode in the transition state from (0,0, 1) to (1, 1, 0) (step 1).

[0080] (j) Mode 10: Regenerative mode in the transition state from (0,0, 1) to (1, 1, 0) (step 2).

[0081] (k) Mode 11: Steady mode where (U, V, W)=(1, 1, 0).

[0082] Moreover, in cases where the control sequence is different fromthe above, the operation of the circuit is similar to that describedabove.

[0083] Furthermore, in the waveform diagram shown in FIG. 6, the modenumber displayed in the bottom row corresponds with the aforementionedmode numbers, and the various waveforms represent signal waveformscorresponding with each of the above modes.

[0084] Next, the operation of the inverter circuit according to thepresent embodiment is described in greater detail based on the notationdefined above for the voltage and the current of each portion, and theon/off state of each of the switching elements.

[0085] Firstly, because in mode 1 (a) the inverter circuit is in thesteady state in which (U, V, W)=(1, 0, 0), the current which flows fromthe DC power source 3 through the IGBT Q1 towards the U phase terminalof the motor 1, flows back from the V phase terminal and the W phaseterminal of the motor 1 and returns to the DC power source 3 through theIGBT Q4 and the IGBT Q6 respectively. Furthermore, in the steady stateof mode 1, the H side switching elements IGBT Q7, Q9 and Q12 of theauxiliary circuit 2B are on, and the L side switching elements IGBT Q8,Q10 and Q11 are off, but because there is no energy stored in theresonant inductance Lr, no current flows to the resonant inductance Lr.

[0086] Next, with the circuit in the state described by mode 1, if theIGBT Q8 and Q11 of the auxiliary circuit 2B are turned on and the IGBTQ7 and Q12 are turned off causing a transition to the state of mode 2(b), then a portion of the electric current flowing from the IGBT Q1 tothe U phase terminal of the motor 1 flows through the resonantinductance Lr and returns to the DC power source 3 via the IGBT Q4 andthe IGBT Q6, meaning the resonant inductance Lr stores the energy due tothe current ILr as an initial electric current.

[0087] When sufficient electric current ILr is stored, and the size ofthe electric current ILr is an amount approximately equal to that of anyone of the load electric currents Iu, Iv or Iw flowing to the motor 1(although there is a positive and negative distinction according to thedirection of the electric current flow, in this case absolute values arecompared), IGBT Q1 and Q6 are turned off, and the mode shifts to theresonant mode of mode 3 (c). Until this point IGBT Q1 and Q6 have beenon, and no voltage has been applied to the snubber capacitors C1 and C6,but the transition to the resonant mode causes the voltages V1 and V6across both sides of the snubber capacitors C1 and C6 to rise, and thecharging of these capacitors begins. However, the voltages V1 and V6across both sides of the snubber capacitors C1 and C6 cannot riserapidly due to the time constant applied by these capacitors, and theIGBT Q1 and Q6 are turned off with the voltage across both sides of thesnubber capacitors C1 and C6 (in other words the voltages V1 and V6across both sides of the IGBT Q1 and Q6) at zero, and consequently ZVSis achieved.

[0088] In the waveform diagram in FIG. 6, when the absolute value of theload electric current Iu or Iv is approximately equal to the electriccurrent ILr, the switching state in the main circuit 2A changes.Moreover, the dotted line for the electric current ILr represents thecomparison of the absolute value with the load electric currents Iv andIw. Furthermore, the ZVS of the IGBT Q1 and Q6 are indicated by thepoints A and B respectively.

[0089] Furthermore, until this point, a voltage similar to the powersource voltage VB had been applied to the snubber capacitors C2 and C5,but in the resonant mode of mode 3, because electrical discharge fromthe snubber capacitors C2 and C5 begins due to the snubber capacitors C1and C6 being connected, the voltages V2 and V5 across both sides of thesnubber capacitors C2 and C5 decrease together with the charging of thesnubber capacitors C1 and C6. The charging current of these snubbercapacitors C1 and C6 and the discharging current of the snubbercapacitors C2 and C5 circulates within the circuit through the resonantinductance Lr, as a resonant current.

[0090] In addition, if this resonant mode continues, further resonantelectric current flows due to the energy stored in the resonantinductance Lr, and when the voltages V2 and V5 across both sides of thesnubber capacitors C2 and C5 reach zero, the energy stored in theresonant inductance Lr then flows via the free wheeling diodes D2 andD5.

[0091] Here, the IGBT Q2 and Q5, which are connected in parallel to thefree wheeling diodes D2 and D5, are turned on, and the regenerative modeof mode 4 (d) is entered. At this time, the IGBT Q2 and Q5 adopt ZVS andare turned on with the voltages across both sides of the snubbercapacitors C2 and C5 (in other words, the voltages V2 and V5 across bothsides of the IGBT Q2 and Q5) at zero. Furthermore, because all of theelectric current flows through the free wheeling diodes D2 and D5 and noelectric current flows through the IGBT Q2 and Q5, the IGBT Q2 and Q5also adopt ZCS and are turned on with the electric current at zero.

[0092] In the waveform diagram in FIG. 6, the ZVS and the ZCS of theIGBT Q2 and Q5 are indicated by point C and point D respectively.

[0093] Furthermore, in the regenerative mode of mode 4, a regenerativeelectric current which flows from the W phase terminal of the motor 1 tothe positive electrode side of the DC power source 3 via the IGBT Q5, aregenerative electric current which flows from the V phase terminal ofthe motor 1 to the negative electrode side of the DC power source 3 viathe IGBT Q4, a regenerative electric current which flows to the U phaseterminal of the motor 1 through the IGBT Q2, and an electric currentwhich flows through the IGBT Q8, the resonant inductance Lr and the IGBTQ11 are generated from the regenerative energy of the motor 1 and theenergy stored in the resonant inductance Lr.

[0094] However, because the power source voltage of the DC power source3 is applied as a reverse voltage to the resonant inductance Lr in orderto reduce the electric current ILr, the electric current ILr graduallydecreases and reaches zero. When the electric current ILr reaches zero,the electric current attempting to flow to the emitter side of the IGBTQ8 and Q11 by means of the power source voltage of the DC power source 3is stopped by the protection diodes D8 and D11, and the steady mode ofmode 5 (e) is entered.

[0095] Next is a description of the operation of shifting from mode 5which is a steady mode in which (U, V, W)=(0, 0, 1), to mode 11 which isa steady mode in which (U, V, W)=(1, 1, 0).

[0096] Firstly, in the steady mode of mode 5, when the IGBT Q7, Q9 andQ12 of the auxiliary circuit 2B are turned on, and the IGBT Q8, Q10 andQ11 are turned off, the sequence shifts to the initial electric currentstorage mode (step 1) of the mode 6 (f). Consequently, a portion of theregenerative current of the motor 1 flowing from the W phase terminal ofthe motor 1 to the IGBT Q5 flows through the resonant inductance Lr andreturns to the motor 1 or the DC power source 3 via the IGBT Q7 and Q9,storing the energy due to the electric current ILr in the resonantinductance Lr as an initial electric current.

[0097] In this state, when the electric current ILr reaches an electriccurrent value exceeding that of the load electric current Iu or Iw, theinitial electric current storage mode (step 2) of the mode 7 (g) isentered, and the electric current which had flowed in the free wheelingdiodes D2 and D5 disappears, and an electric current flows through theIGBT Q2 and Q5 in a forward direction. Here, when the IGBT Q2, Q4 and Q5are turned off, then in a similar manner as was described for theaforementioned mode 3, a discharge current flows to the snubbercapacitors C1, C3 and C6 as a resonant electric current of the resonantinductance Lr, a charging current flows to the snubber capacitors C2, C4and C5, and the resonant mode of mode 8 (h) is entered.

[0098] Moreover, in a similar manner as the IGBT Q1 and Q6 in mode 3,the IGBT Q2, Q4 and Q5 adopt ZVS and are turned off with the voltageacross both sides of the snubber capacitors C2, C4 and C5 (in otherwords, the voltages V2, V4 and V5 across both sides of the IGBT Q2, Q4and Q5) at zero.

[0099] In the waveform diagram in FIG. 6, at the point where theelectric current ILr exceeds the absolute value of the load electriccurrent Iu or Iw, the switching state in the main circuit 2A changes.Furthermore, the ZVS of the IGBT Q2, Q4 and Q5 is indicated by point E,point F and point G.

[0100] In addition, if this resonant mode continues, then in a similarmanner to mode 4, further resonant current flows due to the energystored in the resonant inductance Lr, and when the voltages V1, V3 andV6 across both sides of the snubber capacitors C1, C3 and C6 reacheszero, the energy stored in the resonant inductance Lr then flows via thefree wheeling diodes D1, D3 and D6.

[0101] Here, if the IGBT Q1, Q3 and Q6, which are connected in parallelto the free wheeling diodes D1, D3 and D6, are turned on, then theregenerative mode (step 1) of mode 9 (i) is entered. At this time, theIGBT Q1, Q3 and Q6 adopt ZVS and are turned on with the voltage acrossboth sides of the snubber capacitors C1, C3 and C6 (in other words, thevoltages V1, V3 and V6 across both sides of the IGBT Q1, Q3 and Q6) atzero. Furthermore, because all of the electric current flows to the freewheeling diodes D1, D3 and D6, and no electric current flows to the IGBTQ1, Q3 and Q6, the IGBT Q1, Q3 and Q6 adopt ZCS and are turned on withthe electric current at zero.

[0102] In the waveform diagram FIG. 6, the ZVS and ZCS of the IGBT Q1,Q3 and Q6 are indicated by point H, point I and point J respectively.

[0103] Furthermore, if this state is continued, the electric current ofthe free wheeling diodes D1 and D6 which flowed by means of the energystored in the resonant inductance Lr ceases to flow, and theregenerative mode (step 2) of mode 10 (j) is entered wherein theregenerative current of the motor 1 flows in a forward direction towardsthe IGBT Q1 and Q6.

[0104] In addition, because the power source voltage of the DC powersource 3 is applied as a reverse voltage to the resonant inductance Lrin order to reduce the electric current ILr, the electric current ILrgradually decreases and reaches zero. When the electric current ILrreaches zero, the electric current attempting to flow to the emitterside of the IGBT Q7, Q9 and Q12 by means of the power source voltage ofthe DC power source 3 is stopped by the protection diodes D7, D9 andD12, and the steady mode of mode 11 (k) is entered.

[0105] The operation of mode 1 (a) to mode 11 (k) as shown in FIG. 2A toFIG. 5B in an inverter circuit according to the present embodiment isdescribed above using the example where (U, V, W) are controlled so that(1, 0, 0)−>(0, 0, 1)−>(1, 1, 0). Moreover, when performing spatialvector PWM (Pulse Width Modulation) control in the inverter circuit, theoperation of the inverter circuit in the transition between othercontrol vectors is the same as the above case where (U, V, W) iscontrolled so that (1, 0, 0)−>(0, 0, 1)−>(1, 1, 0), and as such thedescription is omitted.

[0106] Furthermore, FIG. 7 shows a circuit diagram of an invertercircuit according to another embodiment of the present invention,wherein reverse blocking thyristors T1 to T6 are used as the auxiliaryswitching elements of the auxiliary circuit 2B. The theoreticaloperation of the circuit in FIG. 7 is the same as the circuit shown inFIG. 1, but differs in the on/off control of the auxiliary switchingelements, in that the control terminals of the aforementioned IGBTs arevoltage controlled, whereas the control terminals of the reverseblocking thyristors T1 to T6 are current controlled. In the circuit inFIG. 7, it is possible to control large currents depending on thecharacteristics of the thyristors.

What is claimed is:
 1. A resonant inverter circuit comprising: six mainswitching elements which either conduct or are cutoff by means ofswitching control, wherein three sets of two main switching elementswhich comprise each phase of a three phase bridge are connected in thethree phase bridge, and each set of the main switching elements isconnected in series to both terminals of a power source; six freewheeling diodes connected in parallel between two terminals of each ofthe main switching elements; six snubber capacitors connected inparallel between two terminals of each of the main switching elements; athree phase output terminal for connecting a load, connectedrespectively to a connection point of the two main switching elementscomprising each of the sets; a bridge circuit having six auxiliaryswitching elements for causing an electric current to flow in a singledirection, wherein three sets of two auxiliary switching elements areconnected in a three phase bridge, and connection points common to thetwo auxiliary switching elements which comprise each set of theauxiliary switching elements are connected respectively to the threephase output terminal; and a resonant inductance forming a resonantcircuit with the snubber capacitors, connected to an opposite terminalto a terminal connected to the connection point of the auxiliaryswitching elements.
 2. An inverter circuit according to claim 1, whereinone of the auxiliary switching elements of each set of the auxiliaryswitching elements includes a unidirectional switching element whichonly conducts electric current in a direction flowing into theconnection point of the auxiliary switching elements, and anotherauxiliary switching element of each set of the auxiliary switchingelements includes a unidirectional switching element which only conductselectric current in a direction flowing out from the connection point ofthe auxiliary switching elements.
 3. An inverter circuit according toclaim 2, wherein the unidirectional switching elements are elementshaving a withstand voltage greater than a power source voltage of thepower source, in both forward and backward directions.
 4. An invertercircuit according to claim 2, wherein the unidirectional switchingelements are insulated gate bipolar transistors, each auxiliaryswitching element comprises the insulated gate bipolar transistor and adiode, and the diode is either one of a diode in which an anode terminalis connected to a collector terminal of the insulated gate bipolartransistor, and a diode in which a cathode terminal is connected to anemitter terminal of the insulated gate bipolar transistor.
 5. Aninverter circuit according to claim 2, wherein the unidirectionalswitching elements are metal oxide semiconductor field effectstransistors, each auxiliary switching element comprises the metal oxidefield effects transistor and a diode, and the diode is either one of adiode in which an anode terminal is connected to a drain terminal of themetal oxide field effects transistor, and a diode in which a cathodeterminal is connected to a source terminal of the metal oxide fieldeffects transistor.
 6. An inverter circuit according to claim 2, whereinthe unidirectional switching elements are reverse blocking thyristors.